ABSTRACT
Polychlorinated dibenzo-p-dioxins (PCDDs) and poly-
chlorinated dibenzofurans (PCDFs) are two groups of
tricyclic aromatic compounds having molecules with
planar geometry. They are often collectively termed as
‘‘dioxins’’ because of their similar chemical structures,
physical, chemical, and environmental properties and
mechanism of action in biological systems (1). Their
generalized molecular structures are shown in Fig. 1.
Any hydrogen atom at eight carbon atoms numbered
1 to 4 and 6 to 9 may be substituted by a chlorine
atom, which may result in 75 PCDD and 135 PCDF
congeners. The congeners that are of toxicological importance
are substituted at each of the 2-, 3-, 7-, and 8-positions.
Thus, from those 210 theoretically possible congeners,
only 17 are of toxicological concern (Table 1). These
compounds have a similar toxicological profile to that
of the most toxic congener 2,3,7,8-tetrachlorodibenzo-
p-dioxin (see Fig. 1) which is classified by the Inter-
national Agency for Cancer and Research (IARC)
and other reputable organizations as a known human
carcinogen. Dioxin toxic responses include dermal
toxicity, immune suppression, modulation of endo-
crine responses, vitamin A deficiency, carcinogenicity,
reproductive, and developmental and neurobehavioral
defects. Most of these effects, if not all, are mediated
via the aryl hydrocarbon (Ah) receptor present in most
tissues of animals and humans (1-9). Four non-ortho
